Share this:

Press Release

The Royal Swedish Academy of Sciences has decided to
award the Nobel Prize in Physics for 1975 in equal shares to
Professor Aage Bohr, Denmark, to Professor Ben
Mottelson, Denmark and to Professor James Rainwater,
USA, for the discovery of the connection between collective
motion and particle motion in atomic nuclei and the development
of the theory of the structure of the atomic nucleus based on
this connection.

At the end of the Forties nuclear physics had advanced to a stage
where a more detailed picture of the structure of the atom
nucleus was beginning to emerge and it was becoming possible to
assess its properties. The models scientists were working with
then were, however, fairly deficient and contradictory to a
certain extent. The oldest was the drop model, in which the
nucleus was regarded as a drop of liquid, where the nucleons
correspond to the atoms in the liquid. Some properties of the
nucleus, particularly those associated with the "magic numbers",
show however that the individual nucleons definitely affect the
behaviour of the nucleus. This discovery, which is demonstrated
in the scale model, was awarded the 1963 Nobel Prize in Physics.

As time passed it was found that the nucleus has properties,
which cannot be explained by these theories. Perhaps the most
striking were the very marked aberrations from spherical symmetry
in the distribution of charge observed in certain nuclei. It was
also pointed out by several research scientists that this might
indicate that certain nuclei are not spherical but are deformed
as an ellipsoid, but no one could give a reasonable explanation
of this phenomenon

The solution of the problem was first presented by James
Rainwater of Columbia University, New York, in a short paper sent
for publication in April 1950. In this he observes the interplay
between the greater proportion of the nucleons, which form an
inner nucleus, and the outer, the valence nucleons, and he points
out that the valence nucleons can influence the shape of the
inner nucleus. Since the valence nucleons move in a field which
is determined by the distribution of the inner nucleons, this
influence is mutual, If several valence nucleons move in similar
courses, this polarizing effect on the rest of the nucleus can be
so great that the nucleus as a whole becomes permanently deformed
Expressed very simply, it can be said that as a result of their
rotation certain nucleons expose the "walls" of the nucleus to
such high centrifugal pressure that it becomes deformed,
Rainwater also attempted to calculate this effect theoretically
and got results that corresponded with experimental late on the
distribution of the charge.

Aage Bohr, working in Copenhagen, but at this time on a visit to
Columbia University, had, quite independently of Rainwater, been
thinking along the same lines In a paper, sent for publication
about a month after Rainwater's, he formulates the problem in a
more general, but from the physical viewpoint loss lucid,
way

These relatively vague ideas were further developed by Bohr in a
famous work from 1951, in which he gives a very comprehensive
study of the coupling of oscillations of the nuclear surface to
the movements of the individual nucleons. By means of an analysis
of the theoretical formula for the kinetic energy of the nucleus
he could predict the different types of collective excitations:
vibrations obtained by a periodic change of the shape of the
nucleus around a certain mean value and the rotation of the whole
nucleus around an axle at right angles to the symmetry
axle.

Up to then advances made had been purely theoretical and the new
ideas largely lacked experimental foundation. The very important
comparison with experimental data was done in three works,
written jointly by Bohr and Mottelson, and published in the years
1952-1953. The most spectacular finding was the discovery that
the position of energy levels in certain nuclei could be
explained by the assumption that they form a rotation spectrum.
The conformity between theory and experiment was so complete that
there could be no doubt of the accuracy of the theory. This gave
stimulus to new theoretical studies, but above all to very many
experiments to prove the theoretical predictions. This dynamic
development very soon led to a deepened understanding of the
structure of the atomic nucleus.

In the research done since then, Bohr and Mottelson have been
central figures, and have definitely inspired research in this
field, although they themselves have not published many works.
However, when it comes to principles, perhaps the most important
discovery during this period originates from them (in
collaboration with Pines). This concerns the fact that nuclear
matter has properties reminiscent of superconductors.